A known internal combustion engine can use a blended fuel that is a mixture of gasoline and alcohol such as ethanol or methanol. FIG. 10 is a chart showing a relationship between a distillation rate and a temperature of E80 (a blended fuel with 80% ethanol), E20 (a blended fuel with 20% ethanol), and E0 (100% gasoline). Gasoline is composed of multiple components including one having a low boiling point which contributes to an outstanding vaporization characteristic even at low temperatures. Alcohol, on the other hand, is composed of a single component and thus has a fixed boiling point which is high (about 78° C. for ethanol). As is known from FIG. 10, therefore, a blended fuel having a high concentration of alcohol, such as E80, has a drawback that the fuel is extremely difficult to vaporize at temperatures lower than the boiling point of alcohol. Note that a blended fuel having a relatively low alcohol concentration, such as E20, may actually more readily vaporize than 100% gasoline because of azeotropic phenomenon.
For the reasons as described above, when a blended fuel having a high alcohol concentration is used, substantially only the gasoline component vaporizes of the blended fuel injected from a fuel injector during cold starting of the internal combustion engine, with very little of the alcohol component vaporizing. This results in an insufficient amount of vaporized fuel that contributes to combustion, thus posing a problem of tendency toward poor startability. In addition, the starting relies only on the gasoline component of the blended fuel injected, so that a large amount of fuel needs to be injected at starting in order to compensate for the insufficiency. An amount of alcohol component many times the amount of gasoline component that has contributed to combustion fails to vaporize and burn, flowing past a combustion chamber into an exhaust path in a form of HC. This results in a problem in that the amount of HC discharged into the atmosphere tends to be extremely large during cold starting.
JP-A-2009-150397 discloses a fuel apparatus that incorporates a fuel separation membrane for separating a blended fuel into a gasoline component and an alcohol component, and two fuel injectors for each cylinder, a first fuel injector for injecting the gasoline component and a second fuel injector for injecting the alcohol component. In this apparatus, the fuel separation membrane is disposed inside a first fuel rail assembly that communicates with the first fuel injector of each cylinder. The alcohol component separated by the fuel separation membrane element is sent to a second fuel rail assembly that communicates with the second fuel injector of each cylinder. The apparatus is thus capable of injecting only the gasoline component during starting, so that the above problem can be solved.